1. Field of the Invention
The present invention relates to an electromagnetic proximity sensor and, in particular, to an electromagnetic proximity sensor which detects any low frequency clutter component generated in an electromagnetic wave by the approach or movement of a human being or any other object.
2. Description of the Related Art
An electromagnetic proximity sensor, which utilizes electromagnetic waves to detect the approach or movement of a human being or a metal object, is used, for example, in automatic doors of convenience stores or in car security systems. This electromagnetic proximity sensor radiates an electromagnetic wave in the microwave range into the space around it, and detects any low-frequency clutter component generated in the electromagnetic wave when a human being, a metal object, etc. approaches it or moves within an area around it, thereby detecting the approach or movement of such an object.
FIG. 7 is a circuit diagram showing an example of an electromagnetic proximity sensor for a car security system, which sensor is installed at a predetermined position inside a vehicle. The reference numeral 1 indicates a DC regulated power supply circuit, which is adapted to produce a stabilized voltage of +5V upon a +12V input. The reference numeral 2 indicates an electromagnetic wave radiating section, which has a GaAsFET 2a for oscillating microwaves, an oscillation coil 2b consisting of a distributed constant circuit, etc. This electromagnetic wave radiating section 2 effects oscillation in a high-frequency range, for example, of 2.45 GHz. The oscillation coil 2b then serves as an antenna and radiates an electromagnetic wave into the secured area around the vehicle to be protected. When a person approaches the vehicle, a low frequency clutter is generated in the radiated electromagnetic wave. This clutter causes the antenna load of the electromagnetic wave radiating section 2 to fluctuate, with the low-frequency clutter component being carried by the collector voltage of the GaAsFET 2a. The reference numeral 3 indicates a clutter component extractor, which includes a detection resistor 3a for detecting any fluctuation in the collector voltage of the GaAsFET 2a, a capacitor 3b for cutting DC current, an amplifier circuit 3c for amplifying the fluctuating component of the collector voltage, and a low-pass filter 3d for extracting the low-frequency clutter component carried by the collector voltage of the GaAsFET 2a to emit a low frequency clutter component signal. The reference numeral 4 indicates a reference level generating section, and the reference numeral 5 indicates a level comparing section, where the extracted electromagnetic wave clutter component signal is supplied to be compared with a predetermined reference level. When a low frequency clutter component signal of a level higher than the reference level has continued for a certain period of time, the level comparing section 5 emits a sensor signal. This level comparing section 5 includes: a first comparator circuit 5a, which compares the low frequency clutter component signal extracted by the clutter component extractor 3 with a predetermined first reference level and emits an H level signal as long as the low frequency clutter component signal is beyond the first reference level; a reverse blocking diode 5b; an integration circuit 5c, which integrates the output of the reverse blocking diode 5b; and a second comparator circuit 5d, which emits an H level signal when the output of the integration circuit 5c has exceeded a predetermined second reference level, turning on an open collector transistor 5e to emit an L level sensor signal Q.
FIG. 8 shows the operational waveforms in the level comparing section 5. The sensor signal Q is only emitted when the input of a low frequency clutter component signal which is at a level beyond the first reference level has continued for a certain period. When the input is of a short duration, no sensor signal is emitted.
The collector side of the open collector transistor 5e is connected through a cable to a pull-up input resistor in a security system body (not shown) installed at a predetermined position inside the vehicle. When the open collector transistor 5e is turned on, an L level sensor signal Q is supplied to a security control unit to effect a predetermined alarm control operation.
The optimum sensitivity of the electromagnetic proximity sensor is different for different types of automobiles, so that it is necessary in the prior art to perform sensor sensitivity adjustment in accordance with the automobile type when mounting the sensor on the vehicle.
Conventionally, this sensor sensitivity adjustment has been effected by turning a semi-fixed resistor 3e provided in the amplifier circuit 3c to vary the gain of the amplifier circuit 3c.
Thus, with the prior-art technique described above, the sensor sensitivity adjustment has been effected by means of the semi-fixed resistor 3e of the electromagnetic proximity sensor, which means it has been necessary for the adjusting operator to get into the vehicle to turn this semi-fixed resistor 3e of the electromagnetic proximity sensor and then get out of the vehicle to check whether the alarm can be triggered or not. Since this has to be performed repeatedly, the conventional manner of adjustment is rather tedious.